The invention relates to a circuit arrangement for igniting and operating a lamp comprising                input terminals for connection to a supply voltage source,        a DC-AC-converter coupled to the input terminals and equipped with                    a series arrangement comprising a first and a second switching element and connecting the input terminals,            a control circuit, coupled to respective control electrodes of the first switching element and the second switching element, for generating a periodic control signal for alternately rendering the first switching element and the second switching element conductive and non-conductive,            a load circuit shunting one of the switching elements and comprising a series arrangement of an inductive element and a first capacitive element.                        
Such a circuit arrangement is in common use, more in particular for the operation of fluorescent lamps. Generally, the fluorescent lamp is placed in parallel with the first capacitive element comprised in the load circuit. During the ignition of the lamp the frequency of the periodic control signal has a value for which the amplitude of the voltage across the capacitor (and thus across the lamp) is comparatively high to enable ignition of the lamp. As a consequence the amplitude of the current flowing through the series arrangement of the inductive element and the first capacitive element comprised in the load circuit is also comparatively high. This comparatively high amplitude of the current often causes the inductive element to saturate to a certain extent. In case the DC-AC-converter is a self-oscillating circuit, the control signal is often derived from the current through the inductive element. The conductive switching element is rendered non-conductive when the amplitude of the current through the inductive element reaches a predetermined value. Because this way of controlling the switches is generally comparatively fast, the (partly) saturating of the inductive element does not render the generation of the ignition voltage unstable.
In case the DC-AC-converter is not a self-oscillating circuit and the control signal is generated by means of a separate circuit part often comprising an integrated circuit, the ignition voltage is often generated by adjusting the frequency of the control signal at a predetermined value. In case no saturation of the inductive element takes place and the DC-AC-converter is operated inductively, a decrease in the frequency of the control signal corresponds to an increase in the amplitude of the ignition voltage. In case, however, saturation of the inductive element does take place, this saturation causes the inductance of the inductive element to decrease and therefore the resonance frequency of the load circuit to increase. As a result the saturation of the inductive element causes the relation between the frequency of the control signal and the amplitude of the ignition voltage to reverse. Consequently, in case the DC-AC-converter is not a self-oscillating circuit a dependable control of the amplitude of the ignition voltage by controlling the frequency of the control signal is often impossible, when saturation of the inductive element takes place. Some control circuits are equipped with means to measure the current through the conducting switching element or through the inductive element. Switching takes place when the amplitude of the measured current reaches a predetermined value. A disadvantage of this approach is that the switching element can only be rendered non-conductive before or ultimately at the maximal value of the amplitude of the current through the switching element or the inductive element. However, the slight saturation of the inductive element may cause a substantial amount of damping of the ignition voltage, this damping in turn necessitating the switching element to be rendered conductive only after the amplitude of the current through the switch or the inductive element has reached its maximal value. Consequently, switching when the measured current reaches a predetermined value does not result in a dependable control of the ignition voltage.